Adaptive Acoustic Metamaterials Based on Magnetorheological Elastomer and Magnetofluid for Vibration Control

This Ph.D. thesis focuses on advancing the field of acoustic metamaterials by integrating smart materials, particularly magnetorheological elastomers (MREs), to enhance vibration control capabilities. Over recent decades, acoustic metamaterials have emerged as a promising solution for vibration isolation and noise reduction, utilizing their unique ability to create bandgaps where vibrations and waves are significantly attenuated. While traditional acoustic metamaterials have seen success in creating narrow and relatively high-frequency bandgaps, applications often demand for wider and lower-frequency bandgaps, especially in fields such as civil engineering, automotive, and precision equipment where low-frequency vibrations can cause significant issues. This research seeks to address these limitations through the novel use of MRE-based acoustic metamaterials, which offer tunable stiffness properties, thereby enabling adaptive vibration control in real-time.

One of the primary contributions of this thesis is the design and experimental validation of several MRE-based acoustic metamaterial isolators that use semi-active control to adjust bandgap properties dynamically. In these designs, MREs have tunable stiffness which can be modified when different external magnetic fields are applied, allowing the bandgap frequency ranges to be controlled in real-time. Negative stiffness elements, achieved through opposing permanent magnets, help to achieve the low-frequency vibration isolation of the MRE acoustic metamaterial isolator. According to the nonlinearity of the magnetic force, structural stiffness at low amplitudes is relatively low, which is better for the vibration suppression, while the stiffness can be larger at higher amplitudes which is suitable for the maintenance of structural stability. Additionally, by introducing negative stiffness structures and inerters, the thesis demonstrates that the bandgap can not only be shifted to lower frequency ranges but also widened. The inerters within the acoustic metamaterial system introduce additional masses to the resonators without significantly increasing self-weight. This feature contributes to achieving lower-frequency bandgaps while maintaining the stability of the system.

Additionally, this thesis explores the potential of magnetofluid (MF) based acoustic metamaterial for actively acoustic wave manipulation. MFs are fluids that respond to magnetic fields and alter their shape. Under varying magnetic field strengths and orientations, the shape of the MF changes, which can effectively affect the distribution of the acoustic field and create new energy bands for the acoustic metamaterial. With varying volumes of MF, different magnetic field strengths, and directional magnetic poles, experiments demonstrate that the metamaterial can achieve diverse acoustic wave manipulation effects, including selective wave guidance and concentration. By introducing designed structural defects within the acoustic metamaterial, the research demonstrate that acoustic waves can be guided or concentrated, which allows for applications requiring directed sound waves or focused acoustic energy.

The findings of this research not only contribute to the application of acoustic metamaterials in practical engineering fields, but also highlight the potential of MREs and MFs as smart materials for tunable acoustic metamaterials. This work paves the way for further advancements in acoustic metamaterial research, particularly in achieving lower and broader bandgaps. It also contributes to the development of tunable acoustic metamaterials capable of real-time adaptation to changing environmental conditions and varying vibration frequencies.